1. Field of the Invention
The present invention relates generally to the spray-deposited production of a product on a moving substrate by the process of spray casting and, more particularly, is concerned with an improved method and apparatus for spray casting to reduce porosity.
2. Description of the Prior Art
A commercial process for production of spray-deposited, shaped preforms in a wide range of alloys has been developed by Osprey Metals Ltd. of West Glamorgan, United Kingdom. The Osprey process, as it is generally known, is disclosed in detail in U.K. Pat. Nos. 1,379,261 and 1,472,939 and U.S. Pat. Nos. 3,826,301 and 3,909,921 and in publications entitled "The Osprey Preform Process" by R.W. Evans et al, Powder Metallurgy, Vol. 28, No. 1 (1985), pages 13-20 and "The Osprey Process for the Production of Spray-Deposited Roll, Disc, Tube and Billet Preforms" by A.G. Leatham et al, Modern Developments in Powder Metallurgy, Vols. 15-17 (1985), pages 157-173.
The Osprey Process is essentially a rapid solidification technique for the direct conversion of liquid metal into shaped preforms by means of an integrated gas-atomizing/spray-depositing operation. In the Osprey process, a controlled stream of molten metal is poured into a gas-atomizing device where it is impacted by high-velocity jets of gas, usually nitrogen or argon. The resulting spray of metal particles is directed onto a "collector" where the hot particles re-coalesce to form a highly dense preform. The collector is fixed to a performing mechanism which is programmed to perform a sequence of movements within the spray, so that the desired preform shape can be generated. The preform can then be further processed, normally by hot-working, to form a semi-finished or finished product.
The Osprey process has also been proposed for producing strip or plate or spray-coated strip or plate, as disclosed in European Pat. Appln. No. 225,080. For producing these products, a substrate or collector, such as a flat substrate or an endless belt is moved continuously through the spray to receive a deposit of uniform thickness across its width.
Heretofore, extensive porosity typically has been observed in a spray-deposited preform at the bottom thereof, the bottom being its side in contact with the substrate or collector. This well known phenomenon, normally undesirable, is a particular problem in a thin gauge product, such as strip or tube, since the porous region may comprise a significant percentage of the product thickness. The porosity is thought to occur when the initial deposit layer is cooled too rapidly by the substrate providing insufficient liquid to feed the inherent interstices between splatted droplets.
Another defect feature often associated with this substrate region is extensive lifting of initial splats which promotes a non-flat surface. The lifting of the splats is a consequence of solidification contraction and distortion arising from the rapid solidification of the splats.
One approach of the prior art for eliminating these problems is preheating the substrate to minimize or reduce the rate of heat transfer from the initial deposit to the substrate so that some fraction liquid is always available to feed voids created during the spray deposition process. However, it is often difficult to effectively preheat a substrate in a commercial spray deposit system because of the cooling effects of the high velocity recirculating atomizing gas. Further, preheating a substrate increases the potential for the deposit sticking to the substrate.
Other approaches have been proposed for eliminating the porosity problem particularly in thin gauge product produced by the above described spray atomization process. Such approaches include the selection of particular materials for the substrate as exemplified in U.S. Pat. Nos. 4,917,170, 4,938,278 and 4,945,973. U.S. Pat. No. 4,966,224 shows a spray-depositing apparatus in which the substrate is orientated to provide that the particles of the leading edge of the spray travel the same or a less distance as the particles in the center to help reduce porosity. U.S. Pat. Nos. 4,901,784 and 4,907,639 describe asymmetrical gas atomizing devices for use in a gas-atomizing spray-depositing apparatus in which the leading edge of the spray pattern travels a shorter distance to the substrate to provide for a higher fraction of liquid in the initial deposits. U.S. Pat. No. 4,926,927 discloses a gas-atomizing spray-deposition apparatus in which overspray is prevented from becoming entrained in the product being formed.